"TWW" wrote: (clip) The
center of the tree on the bowl bottom had bulged out enough to make
the bowl wobble. (clip)
^^^^^^^^^^^^^^^^^^^^^^
I believe that is the reason the bottom is usually turned concave. Wood
moves, even after it is seasoned, as temperature and humidity vary. If you
left the bottom thicker, and then resanded every time the bowl started to
rock, you might eventually reach a condition where it changes from concave
to flat instead of flat to convex.